1. Field of the Invention
The present invention relates to radar timing circuits, and more particularly to precision swept delay circuits for expanded time ranging systems. It can be used to generate a swept-delay dock for sampling radar, time domain reflectometry (TDR) and laser systems.
2. Description of Related Art
High accuracy pulse-echo ranging systems, such as wideband and ultra-wideband pulsed radar, pulsed laser rangefinders, and time domain reflectometers, sweep a timing circuit across a range of delays. The timing circuit controls a receiver sampling gate such that when an echo signal coincides with the temporal location of the sampling gate, a sampled echo signal is obtained. The echo range is then determined from the timing circuit, so high timing accuracy is essential. A beneficial feature for high accuracy is time expansion, whereby the receiver sampling rate is set to a slightly lower rate than the transmit pulse rate to create a stroboscopic time expansion effect that expands the apparent output time by a large factor, such as 100,000. Expanded time allows vastly more accurate signal processing than possible with realtime systems.
A common approach to generate accurate swept timing employs two oscillators with frequencies FT (e.g., a transmit clock frequency) and FR (e.g., a receive clock frequency) that are offset by a small amount FT−FR=Δ. In a ranging application, a transmit dock at frequency FT triggers transmit pulses, and a receive dock at frequency FR gates the echo pulses. If the receive dock is lower in frequency than the transmit clock by a small amount Δ, the phase of the receive clock can smoothly and linearly slip relative to the transmit clock such that one full cycle is slipped every 1/Δ seconds. Typical parameters are: transmit clock FT=2 MHz, receive dock FR=1.99999 MHz, offset frequency Δ=10 Hz, phase slip period=1/Δ=100 milliseconds, and a time expansion factor of FT/Δ=200,000. This two-oscillator technique was used in the 1960's in precision time-interval counters with sub-nanosecond resolution, and appeared in a short-range radar in U.S. Pat. No. 4,132,991, “Method and Apparatus Utilizing Time-Expanded Pulse Sequences for Distance Measurement in a Radar,” by Wocher et al.
The accuracy of the two-oscillator technique is limited by the differential and integral linearity of the phase slip between the two oscillators. The accuracy of the phase slip is not easy to measure accurately and it is also easy to assume it is somehow perfect. Commercial pulse echo radar rangefinders having a claimed accuracy in the millimeter range require error correction look-up tables, which indicates that high accuracy timing systems do not presently exist.
There are many influences that can affect the smoothness of the phase slip, including: (1) oscillator noise due to thermal and flicker effects, (2) transmit-to-receive clock cross-talk, and (3) thermal transients that typically do not track out between the two oscillators. The receive oscillator is typically locked to the offset frequency by a phase locked loop (PLL) circuit, which does a reasonable job when the offset frequency is above several hundred Hertz. Unfortunately, precision long range systems require extremely high accuracy, on the order of picoseconds, at offset frequencies on the order of 10 Hz. A PLL system cannot meet this requirement for the simple reason that the PLL loop response must be slower than 1/Δ, or typically slower than 100 ms, which is far too slow to control short term phase errors between the two clocks.
U.S. Pat. No. 6,404,288 to Bletz et al addresses the problems associated with controlling low offset frequencies by introducing three additional oscillators into a system that can include, for example, seven counters and two phase comparators, all to permit PLL control at higher offset frequencies than the final output offset frequency, which is obtained by frequency down-mixing. This system is too complex for many commercial applications and it does not control instantaneous voltage controlled oscillator (VCO) phase errors and crosstalk.
A need exists for a compact low cost method and precision timing system that instantaneously controls phase slip errors to produce extremely smooth and accurate phase slip rates. The present invention is directed to such a need.